With the recent demand for large-capacity communication, optical fibers have been installed in wide areas. An example of a coated optical fiber used for such an optical fiber includes: a glass fiber; a primary coating layer coating an outer periphery of the glass fiber; a secondary coating layer coating an outer periphery of the primary coating layer; and a colored layer coating an outer periphery of the secondary coating layer.
When a coated optical fiber having such a structure is immersed in water, particularly, in hot water for a long period, the following problem is known to occur in some cases. Specifically, water bubbles are generated in the primary coating layer, and peeling occurs at an interface between the glass fiber and the primary coating layer (the interface being referred to also as a glass/primary interface), so that the transmission loss increases as compared with that before the immersion in water.
To solve this problem, various attempts have been made conventionally. For example, Patent Document 1 discloses that suppression of generation of large water bubbles during immersion in hot water and reduction of the increase in transmission loss are achieved by using, as a primary coating layer, a material having a peeling force adjusted to a predetermined value. Here, the peeling force is measured as follows. First, a glass plate is coated with the material used as the primary coating layer of the coated optical fiber and immersed in hot water. Then, the peeling force for peeling the material from the glass plate at an angle of 90 degrees is measured.
Meanwhile, Patent Document 2 discloses that suppression of generation of large water bubbles during immersion in hot water and reduction of the increase in transmission loss are achieved by adjusting the glass transition temperatures of a primary coating layer and a secondary coating layer of a coated optical fiber to predetermined values, respectively.